Which SSD upgrade to buy for my iMac? Hard drive is failing.

Adding to the pile.

Put an SSD (the 1T EVO if you like) into a USB 3.0 enclosure or get an external USB 3.0 SSD and run your iMac from that instead risking DIY.

Your profile indicated "21.5 inch iMac" -- that you do not need 2.5 -> 3.5 adapter since 21.5 iMacs use 2.5 inch drive.

If you really want to take on these tasks (SSD + memory upgrade) -- I would highly suggest using Apple Authorized Service Providers because the steps are not for casual DIY.

I would suggest OWC (SSD and memory). Samsung SSDs have been reported issues (which I can not verify personally as I used Crucial SSDs for all my Macs).

Good luck

I'm piling on the - "are you sure you want to do this" bandwagon 🤠

I've opened up a fair number of iMacs over the years and its never been something I liked doing but these new thin models are something else. The LCD is held on with double sided tape and it is a pain to remove. Suction cups really are a must and the little tool the macsales tech uses in the video Allan linked to works tons better than a plastic credit card. Once you are inside it isn't too bad but then you have to get rid of all the mess left from the old tape and apply the new tape and then finally put the LCD back on. The first time I did that I got the LCD just the least bit crooked. The tape grabbed on for dear life and there was no easy way back. Two days later that tiny 1/8" (maybe just 1/16" gab) was so obvious that I just had to redo it.

Are you aware that model requires special opening and resealing procedures? Look at this video and decide if this is something you do yourself or instead opt for professional installation:

https://eshop.macsales.com/installvideos/imac_21inch_late_2013_hdd/iMac14-1/